A prior power drive unit has used two separate hydraulic systems, each having a fixed displacement motor with the output shafts thereof connected to a speed summing gear train. In such a unit, one of the motors is normally operable with the other motor in a standby mode, and if there is a failure in one hydraulic system, the other hydraulic system can take over. In an emergency situation, there is a manual override feature to place both motors in operation and through the speed summing gear train, the flight control actuation system operates at approximately two times its normal rate of speed. This power drive unit (as opposed to a fixed displacement torque summing gear train configuration) conserves hydraulic power as the flow demand of each of the hydraulic motors remains constant in order to double the output speed of the power drive unit while maintaining constant output torque.
It is also known to use a torque summing gear train with plural fixed displacement hydraulic motors to drive a member.
A speed summing gear train is power efficient. The speed summing gear train primarily takes two or more inputs such as the inputs from two hydraulic motors and sums the velocity of these inputs to obtain a single output. The torque of the output remains constant regardless of the number of inputs. As each input is added or subtracted, the gear ratio changes to, in effect, increase or decrease the ratio from the motor to the output such that constant torque remains on the output when multiplied by the number of motors. However, the velocity either increases or decreases as a motor is added or subtracted because of the ratio change.
An example of a speed summing gear train is a simple differential gear train with one motor connected to the sun gear and a second motor connected to the ring gear. With the sun gear being driven and the ring gear not driven by the other motor and locked, the planetary carrier of the differential (on a 2:1 ratio differential) would move at one-half the speed of the sun gear while increasing the torque on the sun gear by 2:1. Conversely, with the ring gear being driven and the sun gear locked, the planetary carrier would again move at twice the speed of the motor driving the ring gear, and with twice the torque. With both motors operating, the planetary carrier will operate at the same speed as the ring gear and the sun gear and there will be no gear ratio. Therefore, there will be no multiplication of torque, however, there will be no reduction in speed from the motors to the output. Therefore, you have the torque of two motors which is the same as one motor with a 2:1 ratio through the gear train; however, the output speed from the differential is motor speed rather than one-half of motor speed.
A prime advantage resulting from the use of the speed summing gear train is that a given load can always be moved no matter how many motors are driving and the motors will consume a given amount of hydraulic power or electric power per motor when moving a load. Output speed can be increased or decreased by adding motors or subtracting motors. If the drive of one motor is lost instantaneously, the ability to drive the output is not lost, but there is a loss of one-half the speed of the output member (in a two motor configuration).
In a torque summing gear train, the gear train is very simple with a single gear accepting the inputs from two or more motors. Torque on the single gear then becomes the sum of the torque on all of the motors. Speed of the single gear does not change by adding or subtracting more motors, but only the torque changes, hence the term, torque summing gear train. The primary advantage of the torque summing gear train is that you do not lose control of the load when there is a mechanical failure in one of the inputs to the single gear because the input from the other motor is connected directly to the gear. There is no requirement for grounding of the failed system such as by use of a brake to react to torque from the motor that is still operating as is required in a speed summing gear train.
The principal disadvantage of the torque summing gear train (requiring a given torque output) is that each input to the summing gear is required to provide the torque and the speed for normal operation; therefore, each motor must be capable of running at full speed and full torque, thereby consuming twice as much flow (in a two motor system) as a speed summing system using two motors during normal operation. Therefore, in the torque summing gear train when multiple inputs are operating, all inputs except one are being wasted and are consuming power from the power supply without any use thereof.
In the speed summing gear train, all of the inputs are consuming the same proportion of the power from the power supply insuring drive of the load at a given force and at the maximum demanded rate during normal operation. When one motor is lost, for whatever reason, the load can still be moved at a lesser rate; therefore, in normal operation the speed summing gear train is far more efficient than the torque summing gear train.